1. Field of the Invention
The present invention relates to tunable wavelength filters and, more specifically, to hitless tunable wavelength filters. A hitless tunable wavelength filter is a filter that can be tuned from a first wavelength to a second non-adjacent wavelength without affecting (reflecting or distorting) any of the through-channel transmitted wavelengths and without introducing any switching transients in the transmitted wavelengths.
2. Technical Background
Tunable filters, e.g. fiber Bragg gratings (FBGs), have been utilized in a number of optical systems to selectively add and/or drop wavelengths (i.e., channels) at appropriate locations in wavelength division multiplexed (WDM) optical systems. As is well known to one of ordinary skill in art, a tunable FBG is a narrow band reflective element which can be tuned on or off an International Telecommunications Union (ITU) standard wavelength such that the wavelength may be reflected by or transmitted through the FBG. In this manner, FBGs act as selectable notch band stop filters which substantially reflect received signals within a range of wavelengths and which substantially pass signals which are not within the range of wavelengths. An ideal FBG reflects one signal and passes the remaining signals substantially unattenuated.
In a typical optical system, the addition or subtraction of an optical signal channel (i.e., a specific wavelength) has been achieved by a controller, which controls a given FBG between one of a transmissive and a reflective state. In such a system, there has typically been a FBG for each channel of the WDM signal and the FBGs have been actuated between a transmissive and reflective state in a number of ways. For example, the period of the fiber Bragg grating may be changed by applying a physical stress to the fiber through the use of an actuator, such as a piezoelectric device. In this manner, adjusting the power applied to the piezoelectric device, via a controller, causes the range of wavelengths reflected by the grating to change.
Alternatively, the effective refractive index of the fiber Bragg grating may be thermally tuned such that the wavelength reflected by the grating varies with temperature. In this manner, the temperature of each grating is adjusted by applying an appropriate amount of power to a heater, which is typically made from an electrically resistive coating that is in thermal contact with the grating. In such systems, the gratings have typically been calibrated such that a given grating reflects a given wavelength at a given temperature. However, in such systems, the ability to switch a grating from one wavelength to another in a hitless manner is limited. In the case of a glass fiber grating, both the ability to stretch the fiber and to change its refractive index with temperature is limited. It is difficult to tune a FBG in a hitless manner from a first wavelength to a second non-adjacent wavelength without affecting intermediate channels. There can also be limitations to switching (tuning) speeds. In temperature controlled systems, the switching speed limitation has generally been attributable to a grating associated thermocouple, which is located near the grating to sense the grating temperature. A controller, coupled to the thermocouple, monitors the temperature reported by the thermocouple and adjusts the power delivered to an associated heater accordingly. However, the temperature reported by the thermocouple typically differs, at least after an initial change, from the temperature of the heater. As such, the controller may overshoot or undershoot a desired heater temperature multiple times before stabilizing on a desired temperature and, thus, experience difficulty in locking onto a desired wavelength and may cause switching transients within the optical system.
What is needed is a practical closed-loop control system that is capable of maintaining a tunable wavelength filter locked to a desired wavelength. It would also be desirable for the system to be able to switch the tunable wavelength filter from one wavelength to another wavelength in a reliable relatively efficient hitless manner, covering a wide range of wavelengths and performing the wavelength shift in an acceptably short period of time.
One embodiment of the present invention is directed to a wavelength selective optical device that includes a first thermo-optic switch (TOS), a second TOS, a first waveguide, a second waveguide, a third waveguide, a heating element and a control unit. The first TOS includes a first, second and third port and the first port of the first TOS receives a wavelength division multiplexed (WDM) signal. The second TOS includes a first, second and third port and the first port of the second TOS provides at least one channel of the WDM signal. The first waveguide is coupled between the second ports of the first TOS and the second TOS. The second waveguide includes a tunable filter formed in the second waveguide that reflects a selected channel from the received WDM signal and is coupled between the third ports of the first TOS and the second TOS. The third waveguide includes a reference filter formed in the third waveguide that receives a reference signal and provides an indication signal. The heating element is in thermal contact with the tunable filter and the reference filter. The control unit is coupled to the heating element, the first TOS and the second TOS and varies a temperature of the heating element responsive to the indication signal provided by the reference filter to adjust the selected channel of the tunable filter. The control unit also controls the switching of the first TOS and the second TOS such that the received WDM signal is routed through the first waveguide when the temperature of the heating element is adjusted.
An alternative embodiment of the present invention is directed to a wavelength selective optical device that includes a first thermo-optic switch (TOS), a second TOS, a first waveguide, a second waveguide, a heating element and a control unit. The first TOS includes a first, second and third port and the first port of the first TOS receives a wavelength division multiplexed (WDM) signal. The second TOS includes a first, second and third port and the first port of the second TOS provides at least one channel of the WDM signal. The first waveguide is coupled between the second ports of the first TOS and the second TOS. The second waveguide includes a tunable filter formed in the second waveguide that reflects a selected channel from the received WDM signal and is coupled between the third ports of the first TOS and the second TOS. In addition, the second waveguide includes a reference filter formed in the second waveguide that receives a reference signal and provides an indication signal. The heating element is in thermal contact with the tunable filter and the reference filter. The control unit is coupled to the heating element, the first TOS and the second TOS and varies a temperature of the heating element responsive to the indication signal provided by the reference filter to adjust the selected channel of the tunable filter. The control unit also controls the switching of the first TOS and the second TOS such that the received WDM signal is routed through the first waveguide when the temperature of the heating element is adjusted.
Additional features and advantages of the invention will be set forth in the detailed description which follows and will be apparent to those skilled in the art from the description or recognized by practicing the invention as described in the description which follows together with the claims and appended drawings.